Plastic expulsion process for forming hollow tubular products

ABSTRACT

A system and process for gas assisted plastic injection molding, particularly of hollow tubular components. The plastic material is injected into the mold cavity through an apex of a cone-shaped inlet configuration. After the molding pressure is held for a period of time, gas is injected into the plastic material and pressure is again held for a period of time, a valve member is opened allowing plastic to be expelled into a secondary cavity. The flow of the plastic and gas are controlled in an axial direction in order to prevent turbulence. The gas flow axially along the center of the tubular member allows the expulsion of plastic from the center uniformly along the length of the tube. In an alternative process, the gas is displaced axially into the barrel of the injection molding machine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/902,354, filed on Jul. 10, 2001, and entitled “PlasticExpulsion Process.”

TECHNICAL FIELD

The present invention relates to the field of injection molding ofplastic materials, and in particular to gas assisted molding.

BACKGROUND OF THE INVENTION

Injection molding of plastic materials is well known and widelypracticed as a means of manufacturing an ever-increasing diversity ofplastic components for industrial and consumer use. During the last twodecades, versions of the process globally referred to as “gas assistedmolding” have been developed and used to overcome some of the problemsinherent in conventional molding, and to reduce costs and improve thequality of the final products.

In conventional gas molding, a gas, such as nitrogen, is injected intothe molten plastic material after it has entered the mold. The lowviscosity gas flows into the paths of least resistance within the moreviscous plastic, thereby forming hollow channels within the plastic. Theprocess is particularly beneficial for thick section moldings, such ashandles, and weight savings of up to 45% or more can result. Also, themolding time cycles can be substantially reduced. In multi-sectionmoldings, the injected gas tends to flow into the thicker sections,again forming hollow continuous channels through which pressure may betransmitted through the medium of the gas. This adds to the scope of thedesigner and removes some of the design restrictions of conventionalmolding.

One conventional gas assisted plastic molding process partially fillsthe mold cavity with an accurately controlled shot volume of plastic.Gas is then injected in order to continue the flow of plastic so thatthe cavity is filled with plastic and gas. The gas is used to exertoutward pressure on the plastic material forcing it against the moldcavity surfaces, thereby achieving a good replication of the moldsurface on the plastic molded surface. After the plastic has solidified,the gas pressure is reduced, the gas is exhausted to atmosphere, and themold is opened and the part ejected. This is sometimes referred to as a“short shot” process.

In another method, the mold cavity is completely or substantially filledwith plastic material and then instead of injecting or packing moreplastic into the cavity to compensate for the volumetric shrinkage ofthe plastic as it cools and solidifies, gas is injected into the plasticso that the gas expansion compensates for the plastic contraction. Inpractice, the initial gas penetration will continue to expand during thecooling cycle while the plastic is shrinking in volume. This issometimes referred to as a “full shot” process.

In the “full shot” process, it is sometimes difficult to achievesufficient gas penetration along intended gas channels because there isinsufficient volumetric shrinkage of the plastic to provide space forthe gas. In such cases, a method of enabling some plastic to outflowfrom the mold cavity into overflow wells or “secondary” cavities ishelpful in providing space for the gas expansion.

In the “short shot” method, some moldings may also be difficult to fillwith plastic and gas to the extremities of the molded article cavity. Ifthe shot volume is too little, the gas may break through the leadingedge of the plastic material during filling, thereby losing control ofthe gas. If the shot volume is too high, the gas will not reach theextremities of the cavity. Therefore thick section moldings using the“short shot” process can also benefit from an additional displacement ofplastic from the article cavity into an overflow cavity.

A method of at least partially filling the cavity before injection ofthe gas is described in U.S. Pat. No. 5,098,637. However, in order touse the method of this patent successfully, it is necessary toaccurately control the shot volume for both “short shot” and “full shot”methods, because there is no resistance to prevent the plastic fromflowing into and filling the overflow cavity before the gas is injected.

In the “short shot” process, the flow of plastic is temporarily stoppedat the end of the filling sequence, and then typically there is a delayof up to five seconds before injection of the gas urges the plasticforward to complete the filling of the article cavity with plastic andgas and the overflow cavity with plastic. In the “full shot” process,the mold cavity is filled with, or substantially filled with, moltenplastic and the gas is injected to compensate for the volumetricshrinkage of the plastic and to displace plastic into the overflowcavity. In both cases, it is not feasible to apply “packing pressure” bythe molding machine because there is nothing to restrain the furtherflow of plastic. In the “short shot” process, or when nearly filling thecavity in a “full shot” process, there may remain unsightly visiblehesitation lines or marks on the molding surface at the position of thefirst plastic injection.

Another method is disclosed in Japanese Patent Application No. 50-74660,where shut-off valves in runners connect the product cavity and“secondary cavities.” In this application, the mold cavity is filledwith thermoplastic resin and then a core resin or gas is injected intothe cavity while the thermoplastic resin in the mold is expelled fromthe mold cavity. After the thermoplastic resin fills the mold cavity,the core resin or gas is injected while the resin in the mold cavity isexpelled.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved gasassisted molding process for use in the injection molding of plasticmaterials, particularly hollow tubular products. It is another object ofthe present invention to provide an improved gas assisted plasticinjection molding process which accurately controls the volume ofplastic into the mold cavity and secondary cavity.

It is still another object of the present invention to provide a gasassisted plastic injection molding process in which a goodreproducibility of the mold cavity surface on the molded product isproduced and flow or weld marks and hesitation marks are minimized oreliminated. It is a still further object of the present invention toprovide a gas assisted plastic injection molding process which hasimproved gas core out with larger expulsion of plastic, thereby reducingthe weight and molding cycle time and securing more consistent wall orskin thickness.

It is still another object of the present invention to provide a gasassisted injection molding process which is suitable for use with a widerange of plastic materials, including gas-filled nylon and other filledmaterials and is also suitable for multi-product cavity molds. It is astill further object of the present invention to provide a process forforming hollow tubing components, particularly those which aredimensionally stable.

The present invention provides a gas-assisted plastic injection moldingprocess which meets the above objectives and provides an improvedprocess for molding plastic products. In accordance with the presentinvention, molten plastic material is first injected into the moldcavity. One or more secondary cavities are positioned adjacent the moldcavity and each is provided with a certain volume. It is generallynecessary to adjust the volume of the cavities after the first orsubsequent mold test trial in order to match the volume of eachsecondary cavity with the volume of plastic required to be expelled fromthe molding cavity. This may be done by metal removal from the mold toincrease the volume, or addition of metal to reduce the volume. Shutoffvalves are positioned between the mold cavity and secondary cavities.The plastic material in the mold cavity is pressurized by the moldingmachine immediately after filling. The “packing pressure” from themolding machine is maintained for a predetermined time period.

Thereafter, gas is injected into the molten plastic to continuepressuring the plastic material in the mold cavity. Again, after apredetermined time period to allow pressurization of the gas to pressthe plastic material against the surfaces of the mold cavity, the valvespositioned between the mold cavity and the secondary cavities are openedenabling the expulsion of a volume of plastic sufficient to fill themold cavity or cavities. Initially, the volume of plastic expelled isdetermined by the time at which the valves are opened. If the valveopening time is prolonged, more of the plastic solidifies in the moldcavity, i.e. the solid skin of the plastic will thicken, and less isexpelled; conversely if the valve opening time is advanced, i.e. thedelay in opening is reduced, the skin thickness is reduced and moreplastic is expelled. When the balance of secondary cavity volume andvalve opening time is optimized, the process can be operatedconsistently shot-after-shot in production. This precisely controls thevolume of expelled plastic material. The gas pressure is again held inthe mold cavity until the plastic material cools and solidifies.Thereafter, the gas is vented or exhausted from the mold cavity, themold is opened, and the part is removed or ejected.

The volume of plastic material which is expelled from the mold cavityinto the secondary cavity is not dependent on the timing of the gasinjection. Instead, the volume of plastic expelled is dependent on thevolume of the secondary cavity or cavities.

In an alternate embodiment, at least two secondary cavities areprovided. Each are connected to the mold cavity with a separate valve.Each of the secondary cavities is connected to the article mold cavityby a flow runner or conduit in which there is a valve member. Theoperation of each of the valves is independently controlled and timed.The opening of the valves can be sequentially timed to optimize therequired expulsion of plastic from the mold cavity and from differentpositions in the mold cavity.

In another embodiment of the invention, the plastic expelled by the gasis forced or drawn back into the injection machine barrel eitherseparately or in combination with one or more secondary cavities.

In a still further embodiment, the opening and closing of the valvemembers may be operated to allow the flow of plastic from the moldcavity to the secondary or overflow cavities by the application of aselected pressure exerted by the injection of gas. The injection of thegas would transmit pressure to the plastic material, which in turn willopen the valve members and overcome a preset closing force. A presetclosing force may be applied to the valve member by mechanical springs,pneumatic mechanisms, hydraulic mechanisms, electrical mechanisms, orthe like.

The opening of the valve members may be controlled in any conventionalmanner, such as mechanical, pneumatic, hydraulic, electric, or othermeans. The control can be either digital or computer timed and can beexternal or integral with the gas pressure control means.

In order to form high quality hollow tubular components, the flow ofplastic and gas are controlled in the axial direction. The gas flowsessentially along the tubular section and expels plastic materialuniformly throughout the length of the tube. The plastic can be injectedfrom a thinner section at the apex of a cone configuration which allowsthe plastic flow to expand gradually up to the maximum requiredcylindrical section. The plastic itself is preferably fed through aperipheral film gate from a runner surrounding the gate section feedinginto the apex of the cone. Also, the speed of the plastic injection iscontrolled from an initially slow rate and increasing to a faster rateafter the cone is filled and as the plastic completes the filling of thecylindrical section. Once the control of the gas injection speed and thepressure is achieved, the valve is opened to the secondary cavityallowing the plastic in the center of the tubular component to beexpelled. With this embodiment, it is also preferable for the distal endof the tubular section to have a cone-shaped configuration whichgradually reduces the flow of plastic to a thinner section. This allowsthe expulsion of the plastic to be controlled through the outflow runnerand stop valve.

Other objects, features, and benefits of the present invention willbecome apparent from the following specification when viewed togetherwith the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate a sequence of steps in a preferred embodiment ofthe present invention;

FIG. 2 is a graph further illustrating the in mold pressure timesequence embodiment of the present invention as set forth in FIGS.1A-1E;

FIGS. 3A-3E illustrate an alternate embodiment of the present invention;

FIGS. 4A-4F illustrate an alternate embodiment of the invention in whichtwo secondary cavities are utilized;

FIGS. 5A-5F illustrate a sequence of steps in another preferredembodiment of the present invention, particularly used for themanufacture of hollow tubular components; and

FIGS. 6 and 7 illustrate alternate embodiments at the entrance end ofthe hollow tubular component embodiment.

DESCRIPTION OF THE EMBODIMENT(S)

FIGS. 1A-1E illustrate the sequence of steps forming a preferredembodiment of the present invention. This process is referred togenerally by the reference numeral 10 in the drawings. In FIG. 1A, aquantity of molten plastic material 20 is positioned in the barrel 22 ofan injection molding machine (not shown). The injection molding machinecan be of any conventional type and can expel the plastic material fromthe nozzle 24 of the barrel in any conventional manner, such as by ahelical screw or a piston 26. Although a piston member 26 is shown andillustrated and described in FIGS. 1A-1E, it is to be understood thatany conventional mechanism used to expel the plastic material from abarrel of an injection molding machine can be utilized.

As shown in FIG. 1A, the nozzle 24 of the injection molding machine isconnected to a mold cavity 30. The mold cavity 30 is positioned in aconventional mold which in turn is positioned in a conventional moldingmachine (not shown). A secondary cavity 32 is positioned incommunication with the mold cavity 30. A valve member 40 is positionedbetween the nozzle 24 and the mold cavity 30 while a second valve member42 is positioned in the conduit or runner 44 positioned between the moldcavity 30 with the secondary cavity 32. The valve members 40 and 42 canbe of any conventional type, such as shut-off type valve members, andcan be operated in any conventional manner, such as pneumatically,hydraulically, electrically, etc. Control of the valve members 40 and 42can also be accomplished with any conventional mechanism or system. Forexample, the mechanism can be computer operated so that the valves canbe accurately timed to open and close as desired in accordance with aprespecified injection molding process.

The quantity of plastic material 20 in the barrel 22 of the injectionmolding machine is sufficient to fill or substantially fill the volumeof the mold cavity 30. The plastic material 20 which can be used orutilized with the present invention can be of any conventional type,such as a thermoplastic resin. The plastic material can also be a filledmaterial, that is mixed with a glass or mineral material.

In an alternate embodiment, one or more secondary cavities can beprovided. The secondary cavities are each connected to the mold cavity30 by separate runners. Also, independently controlled separate valvemembers are positioned in each of the runners or conduits between themold cavity 30 and the secondary cavities. In this embodiment, theoverflow cavities can be filled separately and timed in order to allowthe plastic material to be expelled as desired and where necessary inorder to optimize the production process and plastic product produced bythe process.

Referring back to FIG. 1, the second step in the process is shown inFIG. 1B. In this step, the plastic resin material 20 is injected by theinjection molding machine into the mold cavity 30. The plastic materialin the mold cavity is then pressurized by the molding machine (“packingpressure”) for a short period of time, for example 1-5 seconds. Thisinsures good reproducibility of the mold surface on the molded articlein the cavity. For this process step, valve member 40 is opened in orderto allow plastic material to enter the mold cavity 30, but valve member42 is retained in a closed position in order to prevent any plastic fromentering the secondary cavity 32.

At this point, the gas is injected at selected ram and hold pressures.In the schematic embodiment shown, gas is injected through inlet conduit50. Initially, the valve member 42 remains in a closed condition for apredetermined amount of time such that the pressure of the gas in themolten material 20 forces the plastic material against the innersurfaces of the mold cavity, again helping to provide a good surfacefinish on the plastic product. This is shown in FIG. 1C.

Thereafter, as shown in FIG. 1D, the valve member 40 is closed and thevalve member 42 opened. The pressure of the gas 52 in the molten plasticmaterial 20 in the mold cavity 30 causes molten plastic to be expelledfrom the mold cavity into the secondary cavity 32. In this regard, themolten plastic material is preferably expelled from the center of thethicker sections in the molded article through the connecting gates orrunner 44. Thereafter, a gas “hold” or “packing” pressure is maintainedin the mold cavity while the plastic material cools and solidifies. Thistakes on the order of 10-25 seconds or longer, depending on the size andthickness of the molded plastic product.

It is generally necessary to adjust the volume of the cavities after thefirst or subsequent mold test trial in order to match the volume of eachsecondary cavity with the volume of plastic required to be expelled fromthe molding cavity. This may be done by metal removal from the mold toincrease the volume, or addition of metal to reduce the volume.Initially, the volume of plastic expelled is determined by the time atwhich the valves are opened. If the valve opening time is prolonged,more of the plastic solidifies in the mold cavity, i.e. the solid skinof plastic will thicken, and less is expelled; conversely, if the valveopening time is advanced, i.e. the delay in opening is reduced, the skinthickness is reduced and more plastic is expelled. When the balance ofsecondary cavity volume and valve opening time is optimized, the processcan be operated consistently shot-after-shot in production.

The amount of plastic material which is expelled into the overflowcavity can also be controlled by the time at which the valve member 42is opened.

It is also possible to operate the present process without use of avalve such as valve 40 positioned between the injection molding machineand the mold. It is preferable to include a valve such as 40, however,in order to prevent plastic from being forced back into the screwcylinder when pressurized by the gas.

Once the plastic product has sufficiently cooled and solidified, the gas52 is exhausted, for example, back through conduit 50. This is shown inFIG. 1E. The expelled gas can be either collected and reclaimed forfurther use, or expelled into the atmosphere. Mechanisms and systems forexhausting or venting pressurized gas from the interior of the moldedarticle prepatory to opening the mold are described in numerous patentsin the prior art. In this regard, any conventional mechanism or systemfor exhausting or venting the gas from the mold and molded article canbe utilized in accordance with the present invention. At the same timethat the gas is exhausted, the piston or ram 26 of the injection moldingmachine is typically retracted to its rest position to enable ejectionof the molded part and in preparation for another shot of plasticmaterial.

Once the gas is vented or exhausted from the mold cavity, the mold isopened and the finished plastic product is ejected or removed from themold cavity. At the same time, the expelled plastic material 21 in thesecondary cavity 32 is similarly ejected or removed from the mold. Theplastic in the cavity 32 can be reground and reused, if desired, or thecavity itself can be used to form another plastic part for possible use.

In some cases, the use of a nozzle shut-off valve 40 or hot runner valvegate is recommended to prevent reverse flow of plastic back into thebarrel of the injection molded machine. Also, it is preferred to regrindthe runner and plastic material in the secondary cavities to avoid wasteof material. Further, if it is necessary to adjust the volume of thesecondary cavity after initial molding trials, this can be done byremoving or machining metal in order to enlarge the secondary cavity orby adding metal to reduce the size of the cavity.

The positioning of the gas injectors in the mold cavity shouldpreferably be adjacent to the thicker sections and remote from, and atthe opposite extremity to, the expulsion gates and runners. Also, thepositioning of the plastic feed gates in the mold cavity should beselected to optimize the flow of plastic into the mold cavity. It may benecessary to prevent in-mold gas pressure from forcing the plastic backinto the machine with a shut-off valve, by holding the ram screwforward, or by use of a hot runner system with valve gates.

A graph illustrating the sequence of steps described above is shown inFIG. 2. The graph is referred to generally by the reference numeral 60in FIG. 2 and charts the molding cycle time relative to the plasticpressure in the mold. In this regard, the pressure in the mold initiallyincreases as shown by line 62 as the plastic is injected into the moldcavity. The molding machine then holds the plastic packed pressure asshown by line 64. The time at which the machine packs the pressure isindicated by the arrow 66. That time can be adjusted as desired.

Thereafter, as shown by line 68, the gas is injected into the plasticmaterial in the mold cavity. At point 70, the valve member 44 regulatingthe flow of plastic material into the secondary cavity is opened. Thisallows plastic to be expelled from the mold cavity into the secondarycavity. This step is indicated by the arrow 72. Thereafter, the pressureof the gas in the mold cavity is held. This is shown by line 74. Thetime of the gas packing pressure is shown by the arrow 76 in FIG. 2.

Subsequently, the gas pressure is reduced in the mold. This is shown byline 78. The gas pressure can also be held constant for a period oftime, as shown at 80, in order to allow the plastic material to cool andharden. Thereafter, that is once the plastic part is cooled andhardened, it is ejected from the mold. Once the pressure of the gas isvented or exhausted from the mold cavity, as shown by line 82, the moldis opened and the part removed. This is shown at point 84 on the graphof FIG. 2.

With the system and process of the present invention, the method is notdependent on the injection of an accurately controlled shot volume ofplastic repeatedly shot after shot. Also, the ability to pressure packthe plastic while the article cavity is filled with plastic, or isfilled with plastic and initial injection of gas, insures accuratereplication of the mold cavity surface without shrinkage of the plasticfrom the surface.

Avoiding the need to partially fill the article cavity eliminates thetendency to form hesitation marks on the product at positions to whichthe plastic flows and stops before the gas is injected.

The dependence on a fixed volume of plastic material which is expelledfrom the mold cavity eliminates the variable dependence on timing of thegas injection. Also, the present invention is suitable for multi-cavitymolds, with each cavity being able to be connected to one or moresecondary cavities. It is not believed practical to depend on thebalancing of the flow of plastic in order to partially fill each cavity.

The present invention is suitable for a wide range of thermoplasticresin materials, including glass-filled fiber materials which requireearly pressurization in order to achieve acceptable surface finishes.Also, a more consistent and uniform wall section thickness is achievablethroughout the molding channel as a result of more positive control ofthe gas and plastic flows. The process is further operable on moldingmachines which are not capable of accurately delivering consistentvolumes of plastic material.

The position of the gas injectors in the mold cavity is not as criticalas it is with other processes in which overflow cavities are utilized.In this regard, the present invention is suitable for expelling plasticfrom more than one position in an article cavity. Also, the valvemembers into the secondary cavities can be sequentially opened andclosed in order to optimize plastic expulsion and to avoid thickaccumulations of plastic remaining in the molded article.

An embodiment of the invention in which more than one secondary cavitiesare utilized is shown in FIGS. 4A-4F and designated generally by thereference numeral 150. The embodiment 150 includes an injection moldingmachine 152 which is set up to inject a quantity of molten plasticmaterial 154 into a mold cavity 156 in a mold. The primary mold cavity156 has a pair of secondary cavities 158 and 160 which are connected tothe mold cavity 156 by conduits or runners 162 and 164, respectively.Valve members 166 and 168 are positioned on the conduits and act to openand close the flow of plastic from the mold cavity 156 to the secondarycavities. Another valve member 170 is positioned at the inlet to theprimary mold cavity 156 (or in the nozzle) and used to open and closethe conduit 172 which connects the inject molding machine nozzle to themold cavity. Port or pin member 174 is used to inject gas into the moldcavity.

The sequence of steps for use of system 150 is illustrated in FIGS. 4Ato 4F. Plastic material 154A is first injected into the mold andretained there for a brief period of “packing pressure” (FIG. 4B). Gas180 is then injected into the plastic material in the mold and thepressure is held for another period of time (FIG. 4C). Valve member 166is then opened and a first quantity of plastic material 154B is expelledinto a first secondary cavity 158 (FIG. 4D). Thereafter, valve member168 is opened and a second quantity of plastic material 154C is expelledinto a second secondary cavity 160 (FIG. 4E). Once the part has cooledand solidified sufficiently, the gas is exhausted back through port 174(FIG. 4F). In the final steps, the mold is opened, the part is ejected,and the system is prepared for the start of another cycle.

The embodiment shown in FIGS. 4A-4F is representative of any system inaccordance with the present invention in which more than one secondarycavity is utilized. Variations and changes can be made by persons ofordinary skill in the art to develop other multiple secondary cavitysystems. It is often advantageous to expel plastic material into second,third, or even fourth secondary cavities in order to relieve thicksections of the plastic remaining after the first expulsion. This can beevident in a non-uniformly thick section molding such as an automobiledoor handle.

With the present invention, the injection of an unpressurized accurateshot weight or filling volume is not necessary. Also, the timing of thegas injection is not as critical. The packed pressure exerted by themolding machine and subsequently the gas when the cavity is full ofplastic material insures a good reproducibility of the mold cavity andthe molded article itself. Flow or weld marks are reduced. Also, theappearance of hesitation marks, when partial filling of a cavity isdesired, ceases to be a concern with the present invention.

The operation of the valve member 42, as well as any other valve memberswhich are positioned between the mold cavity and the secondary cavities,can be operated independently and timed to be sequential with the othersecondary cavities. This allows the plastic to be expelled from the moldcavity in a desired sequence and in order to allow formation of variouschannels in various thicker sections or members of the product. Also, asnoted, the volume of plastic expelled from the mold cavity is notdependent on the timing of the gas. Instead, it is dependent on thetiming of the valves and the volume of the secondary cavity or cavitiesand the opening and closing sequence of the valve members from the moldcavity into the secondary cavities.

In a further embodiment of the present invention, the valves in therunners or conduits between the mold cavities and the secondary cavityor cavities can be operated in a different manner. The valve members maybe operable by the application of a selected pressure which is exertedby the injection of gas and in turn a transmission of pressure to theplastic material. This in turn will open the valve member, thusovercoming a preset closing force. The preset closing forces may beapplied to the valve members by mechanical spring members or othermeans, such as pneumatic, hydraulic, or electric.

Further, the opening and closing of the valve members may be controlledby any conventional means, such as pneumatic, hydraulic, electrical, ormechanical means. The opening and closing of the valve members can alsobe controlled by external means which may include digital or computertiming, external or integral with the gas pressure control means.

As an alternative to expelling the plastic material into secondarycavities, it is also possible to expel the plastic material back intothe injection molding machine barrel. This is accomplished by the gaspressure pushing back the injection screw or piston against acontrollable back pressure. This process is shown in FIGS. 3A-3E andreferred to generally by the reference numeral 100. As shown in FIG. 3A,plastic material 102 is positioned in a barrel 104 of an injectionmolding machine (not shown). The barrel 104 has a nozzle 106 which isconnected in any conventional manner with a mold cavity 108. A valvemember 110 controls the flow of plastic from the barrel member into themold cavity. At the initial sequence of steps, as shown in FIG. 3B, thevalve member 110 is opened allowing plastic material to be expelled orinjected from the barrel 104 into the mold cavity 108. Thereafter, thepressure is temporarily held by the injection molding machine relativeto the plastic in the mold cavity 108, preferably for at least 1-5seconds.

Thereafter, as shown in FIG. 3C, gas is injected into the plasticmaterial in the mold cavity through gas injection conduit 120. The gas122 expels the plastic back into the machine cylinder and forms a hollowcavity in the plastic material in the mold cavity 108.

At this point, the pressure is reduced from the molding machine screw orplunger member 130 which enables expulsion of molten plastic material102 from the mold cavity back into the machine cylinder or barrel 104.Space in the barrel 104 can be formed by the force of the gas forcingthe plunger member 130 away from the mold.

Following completion of plastic expulsion, the gas pressure is heldduring cooling and solidification of the plastic material in the moldcavity. This is shown in FIG. 3D. At this point, the plastic issubjected to packing pressure and has its surface forced tightly againstthe inside surfaces of the mold cavity. This produces a good surfacefinish and creates a fall definition of the surface of the mold.Thereafter, the gas pressure in the mold cavity is reduced under controlto atmospheric pressure. This is shown in FIG. 3E. Thereafter, themachine barrel 104 is completely refilled with plastic material andready for the next molding cycle. At the same time, the mold is openedand the formed plastic part is removed or ejected from the moldingmachine.

The advantages of this alternate embodiment of the present invention isthat the expelled plastic can be remolded in succeeding shots. Thiseliminates regrinding or recovery of the expelled material from aspillover or secondary cavity. Also, retrimming of the molding is notnecessary, and the system does not have to expend the cost of additionalshutoff valves in the runner members.

The present invention also can be used in the manufacture of hollowtubular components. A preferred molding system and process for this isshown in FIGS. 5A-5F, while other embodiments of portions of thismechanism and system are illustrated in FIGS. 6 and 7. There is a needtoday for the manufacture of hollow tubular components from plasticmaterials. These components can be used, for example, as rollers forprinters. The tubular components have to be dimensionally stable andstraight circular cross-sections with concentric hollow intersections inmany cases. It is also desirable to include features in the moldingwhich could normally not be extruded, but which are possible withinjection molded products. These include grooving or screw threads onthe outer surface, brackets and other external fittings, connectionmechanisms for use as spindles, and the inclusion of gear teeth orpolywheel configurations to provide ways to rotate the roller.

With conventional injection molding techniques, either as a solidsection or with mechanically moving side cores to form the inter-tubularsection, it is difficult to achieve flatness without distortion over thelength of the component and to reduce molding time cycles. In priorattempts to use gas assisted molding for hollow products or components,it has proved difficult to achieve concentricity and uniform wallsection thickness along the length of the tubular components.

In order to achieve axial flatness over the length of the tubularsection, uniform shrinkage of the plastic as it cools and changes from aliquid into a solid should be achieved. It is also important toeliminate or minimize stress in the molding of the plastic since moldingstresses can cause ejected parts to distort or warp. Injection moldedplastic parts which have stress in them have a tendency to relieve thatstress by distortion and warpage of the component after they are ejectedfrom the mold.

With the present invention, concentricity and uniformity of wall sectioncan be achieved. The plastic and gas flows are controlled in an axialdirection, and there is no random or uncontrolled sideway movement orturbulence in the plastic flow. The gas flows axially at the center ofthe tubular section and is utilized to expel the plastic uniformlythroughout the length of the tube into the secondary cavity or cavities.There is no lateral movement of the plastic after forming and within thesemi-molten skin adjacent to the mold cavity surface. A uniform rate ofcooling is achieved since there is good temperature control of the moldcavity members and there is a good plastic to mold surface contact toinsure uniform conduction of heat from the plastic.

In order to achieve a flow of plastic without turbulence, a mold cavityis formed which has a circular middle section and two cone-shapedconfigurations at the two ends. The plastic is injected through aperipheral film gate from a runner surrounding the gate section feedinginto the apex of the cone at the inlet end. The plastic is injectedalong a thinner section and it flows and expands gradually along a coneor cone-like configuration up to the maximum required cylindricalsection.

The gas is injected in a central position along the axis of the tube andused to expel the plastic from the center. The gas is injected near theapex of the cone in order to allow a gradual expansion into the desiredintersection of the plastic. The speed of plastic injection iscontrolled from an initially slow rate and increased to a faster rateafter the cone is filled and as the plastic completes the filling of thecylindrical section. The speed of the plastic injection slows againbefore the final cavity filling is completed.

The control of the gas injection speed, and therefore the pressure, isalso controlled such that the gas is injected at a low pressure beforeexpulsion of the plastic begins. This means that the pressure in whichthe gas is injected is lower before the opening of the valve in therunner or conduit which connects the product cavity to the secondarycavity.

The tubular section at the distal end of the product cavity adjacent tothe secondary cavity can be reduced in size to gradually reduce the flowof plastic to a thinner section. This allows the expulsion of theplastic to be controlled through the outflow runner and stop valve intothe secondary cavity. In this regard, a second cone-shaped cavityconfiguration is shown in the drawings and described below, but it is tobe understood that any shape or configuration can be utilized at thedistal end of the mold cavity. Also, although it is preferred that thecross-sectional size be reduced at the distal end of the cavity, this isnot a requirement of the invention.

A preferred sequence of steps which can be utilized in the plasticexpulsion process in order to manufacture hollow tubular components isshown in FIGS. 5A-5F. The process is generally referred to by thereference numeral 200 in the drawings.

As shown in FIG. 5A, the mold cavity 202 generally has a centralelongated portion or section 204, a cone shaped configuration 206 at theleading end (that is the entrance at which the gas and plastic areinjected into the cavity), and preferably a corresponding cone-shapedconfiguration 208 at the distal end adjacent the secondary cavity 210. Avalve member 212 is positioned in the conduit or runner connecting themold cavity 202 with the secondary cavity 210. The valve member can beany conventional type, such as those described above with reference toFIGS. 1-4.

Plastic is introduced into the mold cavity 202 through runner 220 andfilm gate 222. The plastic feed runner 220 and the plastic gate 222 arebetter shown in FIGS. 6 and 7.

The gas is injected into the mold cavity from gas injector port 230which is positioned at the apex of the cone-shaped inlet section 206.Gas is directed to the port 230 from conduit or inlet line 232.

Also, as shown in FIGS. 6 and 7, it is possible to have a moving core240 as part of the mechanism and system.

As shown in FIG. 5B, the sequence of the molding process includes theinitial injection of plastic material into the mold cavity 202. At thispoint, the valve member 212 is closed. After the plastic is injectedinto the mold cavity, the plastic is pressurized or packed in the cavityin order to provide good surface characteristics of the outer surface ofthe molded component. In this regard, the plastic is pressurized andcompacted for a short dwell period of time, on the order of 1-5 seconds,by the molding machine. This insures good reproducibility of the moldsurface on the molded product or article and good replication of themold cavity shape.

Thereafter, as shown in FIG. 5C, the injection of the gas is commenced.The gas which is shown schematically by the reference numeral 250,continues to apply pressure within the plastic in the mold cavity 202.After a short dwell time in which the plastic pressure is utilized toassist in forcing the plastic against the internal walls or surfaces ofthe mold cavity, the valve member 212 is opened. This is shown in FIG.4D.

With the valve member 212 opened, the molten plastic material in thecenter of the mold cavity 202 is allowed to be expelled from the moldcavity. The expulsion of the molten plastic is taken from the center ofthe tubular section and enters the secondary cavity 210 through theconnecting runner 214. In FIG. 5D, the hollow center of the tubularcomponent, which is filled with gas, is indicated by the referencenumeral 250, while the outer plastic now tubular component is referredto by the reference numeral 252. The plastic material in the overflowcavity 210 is referred to by the reference numeral 252A.

When the expulsion of plastic is complete, the gas pressure within theinner tubular channel is maintained so that the outer surface of theplastic remains in contact uniformly with the mold cavity surface,thereby insuring uniform conduction of heat from the plastic to themold.

After complete solidification of the plastic tubular component, the gaspressure is relieved and the gas is exhausted from the mold. This isshown in FIG. 5E. Thereafter the moveable core (if utilized) is removedfrom the mold cavity and the mold is opened and the part ejected. Theportion of the plastic material 252A in the secondary cavity 210 is alsoejected at the same time. In this regard, as indicated above, theplastic material in the secondary cavity can be reground and used again,can be scrapped, or can be made into another plastic component or parthaving the shape of the secondary cavity.

Once the plastic tubular component is formed and ejected from the mold,post molding operations are necessary in order to trim off the conesections at each end. This forms the final hollow tube or component.

The cone-shaped sections or configurations 206 and 208 of the moldcavity 202 preferably have included angles up to between 10-60 degrees.This assists in the uniform axial fill of the plastic from the apex ofthe cone and allows the plastic to expand up to the maximum cylindricalsection as required in order to avoid turbulence or nonuniform flow ofthe plastic.

Injecting the gas at an axial direction from a gas injection assembly orport 230 mounted in a moveable core assembly 240 enables the gasinjector to be withdrawn axially from the molded component before themold is opened and the part is ejected.

As an additional embodiment, the moveable gas injector and plastic feedgate can be combined in order to feed the plastic to a ring gateconfiguration in which the plastic flows from a ringed runner formedaround the gas injector barrel, and then through a thinner ringed gateflowing into the mold cavity so that it is coaxial with the gasinjection when the gas is injected after the mold filling is complete.This is illustrated preferably in FIG. 7. An alternate configuration ofthe plastic runner can be utilized where the runner is formed by agroove between the enlarged moveable core and the mold plate. This isillustrated in FIG. 6.

It is possible in accordance with the hollow tubular embodiment of thepresent invention to use the alternate version of the plastic expulsionprocess as described above with respect to FIGS. 3A-E. In thisalternative embodiment, the molten plastic is expelled back into themolding machine cylinder, rather than into a secondary cavity. Theexpelled plastic forces back the screw in the barrel of the moldingmachine to provide the necessary controlled space volume within thecylinder.

For this alternate embodiment, the gas is injected from a cone extensionof the tubular component at the opposite end to the plastic feed gatesection. The gas injector is mounted on a moveable side core whichinjects the gas into the mold cavity in an axial direction, therebyexpelling a uniform flow of plastic exiting from the cone-shaped feedsection, through the feed gates, runners, and sprue and back into themachine cylinder. With this alternate embodiment, the pressure is againmaintained in the mold cavity in order to force the plastic against thesurfaces of the mold cavity. As stated above, this insures good servicereplication and uniform conduction of heat from the plastic to the mold.

As stated above, the present invention enables the manufacturer of aninjection molded tubular structure of component. The component is moldedas one piece and can be molded together with additional features, suchas pulley wheels, gear wheels with profiled teeth, or with screwthreads, grooves, or other profiles on the outer surface of the tubularsection. These additional features can be formed by machining applicablecavities for them in the mold cavity surfaces.

While particular embodiments of the invention have been shown anddescribed, numerous variations and alternative embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. A process for injection molding a hollow plastic tubular articlecomprising the steps of: (a) injecting a quantity of plastic materialinto a mold cavity to at least substantially fill said mold cavity, themold cavity having a substantially cone-shaped inlet portion, wallsurfaces forming an elongated central portion, and an exit portion, saidcone-shaved inlet portion having an apex; (b) injecting pressurized gasinto the plastic material in the mold cavity said gas being injected atsaid apex; (c) holding the pressure of the gas and plastic in the moldcavity for a predetermined amount of time; and (d) allowing a portion ofthe plastic material in the mold cavity to be expelled into at least onesecondary cavity coupled to the mold cavity.
 2. The process as set forthin claim 1 further comprising the steps of: (e) permitting the plasticmaterial to solidify; (f) exhausting the gas from the mold cavity; and(g) removing the plastic article from the mold.
 3. The process as setforth in claim 1 further comprising the step of holding constant theplastic material injection pressure in the mold cavity for apredetermined period of time prior to the injection of gas into theplastic material.
 4. The process as set forth in claim 1 wherein saidplastic material is injected into the mold cavity from an injectionmolding machine with a barrel and nozzle, said method further comprisingthe step of allowing a portion of the plastic material in the mold to beexpelled back into the barrel of the injection molding machine.
 5. Theprocess as set forth in claim 1 wherein said exit portion comprises asecond substantially cone-shaped portion, said cone-shaped exit portionhaving an apex and said expulsion of plastic material from the moldcavity into the secondary cavity occurs through said apex.
 6. Theprocess as set forth in claim 1 further calculating the volume of saidat least one secondary cavity in order to allow expulsion of apredetermined amount of plastic material from the mold cavity.
 7. Theprocess as set forth in claim 1 wherein the step of allowing a portionof the plastic material in the mold to be expelled comprises opening avalve member in a conduit connecting the mold cavity with the secondarycavity.
 8. The process as set forth in claim 1 wherein the plasticmaterial is injected into the mold cavity at said cone-shaped inletportion and enters the mold cavity along said wall surfaces thereof. 9.The process as set forth in claim 8 further comprising a ring gatemechanism for injecting the plastic material into said cone-shaped inletportion.
 10. A process for injection molding a hollow plastic tubulararticle comprising the steps of: (a) injecting a quantity of plasticmaterial to fill or substantially fill a mold cavity, the mold cavityhaving a first substantially cone-shaped inlet portion, an elongatedcentral portion and an exit portion, said cone-shaped inlet portionhaving a first apex; (b) injecting pressurized gas through said firstapex and into the plastic material in the mold cavity; (c) holding thepressure of the gas and plastic in the mold cavity for a predeterminedamount of time; (d) allowing a portion of the plastic material in themold cavity to be expelled into at least one secondary cavity coupled tothe mold cavity; (e) permitting the plastic material to solidify; (f)exhausting the gas from the mold cavity; (g) removing the tubular-shapedplastic article from the mold; and (h) trimming at least one end of thearticle to form a tubular article of substantially constantcross-section.
 11. The process as set forth in claim 10 furthercomprising the step of holding constant the plastic material injectionpressure in the mold cavity for a predetermined period of time prior tothe injection of gas into the plastic material.
 12. The process as setforth in claim 10 wherein said plastic material is injected into themold cavity from an injection molding machine with a barrel and nozzle,said method further comprising the step of allowing a portion of theplastic material in the mold to be expelled back into the barrel of theinjection molding machine.
 13. The process as set forth in claim 10wherein said exit portion comprises a substantially cone-shaped portion,said cone-shaped exit portion having a second apex and said expulsion ofplastic material from the mold cavity in the secondary cavity occursthrough said second apex.
 14. The process as set forth in claim 10further calculating the volume of said at least one secondary cavity inorder to allow expulsion of a predetermined amount of plastic materialfrom the mold cavity.
 15. The process as set forth in claim 10 whereinthe step of allowing a portion of the plastic material in the mold to beexpelled comprises opening a valve member in a conduit connecting themold cavity with the secondary cavity.
 16. A process for injectionmolding a hollow tubular plastic article utilizing an injection moldingmachine with a barrel and nozzle and a mold with a mold cavity therein,the mold cavity having a substantially cone-shaped inlet portion, wallsurfaces forming an elongated central portion, and an exit portion, saidmethod comprising the steps of: (a) injecting a quantity of plasticmaterial into said cone-shaped inlet portion of the mold cavity from theinjection molding machine; (b) injecting pressurized gas into theplastic material in the mold cavity; and (c) allowing a first portion ofthe plastic material in the mold cavity to be expelled back into thebarrel of the injection molding machine.
 17. The process as set forth inclaim 16 further comprising the step of holding the constant pressure ofthe gas and plastic material in the mold cavity for a predeterminedamount of time before said first portion of the plastic material isexpelled back into the injection molding machine.
 18. The process as setforth in claim 16 wherein a predetermined amount of plastic material isexpelled back into the injection molding machine.
 19. The process as setforth in claim 16 wherein the gas is injected into the plastic materialfrom said exit portion.
 20. The process as set forth in claim 16 whereinthe plastic material is injected into the mold cavity at saidcone-shaped inlet portion and enters the mold cavity along said wallsurfaces thereof.
 21. The process as set forth in claim 20 furthercomprising a ring gate mechanism for injecting the plastic material intosaid cone-shaped inlet portion.
 22. The process as set forth in claim 16wherein the step of allowing a first portion of the plastic material inthe mold to be expelled back into the barrel of the injection moldingmachine comprises opening a shut-off valve member positioned betweensaid mold cavity and said barrel.
 23. The process as set forth in claim22 wherein said valve member is included as part of the nozzle.
 24. Aprocess for injection molding a hollow plastic tubular articlecomprising the steps of: (a) injecting a quantity of plastic materialinto a mold cavity to at least substantially fill said mold cavity, themold cavity having a substantially cone-shaped inlet portion, wallsurfaces forming an elongated central portion and an exit portion; (b)holding constant the plastic material injection pressure in the moldcavity for a first predetermined period of time; (c) injectingpressurized gas into the plastic material in the mold cavity subsequentto said first predetermined period of time; (d) holding the pressure ofthe gas and plastic in the mold cavity for a second predetermined amountof time; and (e) allowing a portion of the plastic material in the moldcavity to be expelled into at least one secondary cavity coupled to themold cavity.
 25. The process as set forth in claim 24 further comprisingthe steps of: (f) permitting the plastic material to solidify; (g)exhausting the gas from the mold cavity; and (h) removing the plasticarticle from the mold.
 26. The process as set forth in claim 24 whereinsaid cone-shaped portion has an apex and said gas is injected into theplastic material at said apex.
 27. The process as set forth in claim 24wherein said plastic material is injected into the mold cavity from aninjection molding machine with a barrel and nozzle, said method furthercomprising the step of allowing a portion of the plastic material in themold to be expelled back into the barrel of the injection moldingmachine.
 28. The process as set forth in claim 24 wherein said exitportion comprises a second substantially cone-shaped portion, saidcone-shaped exit portion having an apex and said expulsion of plasticmaterial from the mold cavity into the secondary cavity occurs throughsaid apex.
 29. The process as set forth in claim 24 further calculatingthe volume of said at least one secondary cavity in order to allowexpulsion of a predetermined amount of plastic material from the moldcavity.
 30. The process as set forth in claim 24 wherein the step ofallowing a portion of the plastic material in the mold to be expelledcomprises opening a valve member in a conduit connecting the mold cavitywith the secondary cavity.
 31. The process as set forth in claim 24wherein the plastic material is injected into the mold cavity at saidcone-shaped inlet portion and enters the mold cavity along said wallsurfaces.
 32. The process as set forth in claim 31 further comprising aring gate mechanism for injecting the plastic material into saidcone-shaped inlet portion.
 33. A process for injection molding a hollowplastic tubular article comprising the steps of: (a) injecting aquantity of plastic material into a mold cavity to at leastsubstantially fill said mold cavity, the mold cavity having asubstantially cone-shaped inlet portion, an elongated central portionand a substantially cone-shaped exit portion, said exit portion havingan apex; (b) injecting pressurized gas into the plastic material in themold cavity; (c) holding the pressure of the gas and plastic in the moldcavity for a predetermined amount of time; and (d) allowing a portion ofthe plastic material in the mold cavity to be expelled into at least onesecondary cavity coupled to the mold cavity, wherein said expulsion ofplastic material occurs through said apex.
 34. The process as set forthin claim 33 further comprising the steps of: (e) permitting the plasticmaterial to solidify; (f) exhausting the gas from the mold cavity; and(g) removing the plastic article from the mold.
 35. The process as setforth in claim 33 further comprising the step of holding constant theplastic material injection pressure in the mold cavity for apredetermined period of time prior to the injection of gas into theplastic material.
 36. The process as set forth in claim 33 wherein saidplastic material is injected into the mold cavity from an injectionmolding machine with a barrel and nozzle, said method further comprisingthe step of allowing a portion of the plastic material in the mold to beexpelled back into the barrel of the injection molding machine.
 37. Theprocess as set forth in claim 33 further calculating the volume of saidat least one secondary cavity in order to allow expulsion of apredetermined amount of plastic material from the mold cavity.
 38. Theprocess as set forth in claim 33 wherein the step of allowing a portionof the plastic material in the mold to be expelled comprises opening avalve member in a conduit connecting the mold cavity with the secondarycavity.
 39. A process for injection molding a hollow plastic tubulararticle comprising the steps of: (a) injecting a quantity of plasticmaterial into a mold cavity to at least substantially fill said moldcavity, the mold cavity having a substantially cone-shaped inletportion, wall surfaces forming an elongated central portion, and an exitportion, said plastic material being injected at said cone-shaped inletportion and entering the mold cavity along the wall surfaces thereof;(b) injecting pressurized gas into the plastic material in the moldcavity; (c) holding the pressure of the gas and plastic in the moldcavity for a predetermined amount of time; and (d) allowing a portion ofthe plastic material in the mold cavity to be expelled into at least onesecondary cavity coupled to the mold cavity.
 40. The process as setforth in claim 39 further comprising the steps of: (e) permitting theplastic material to solidify; (f) exhausting the gas from the moldcavity; and (g) removing the plastic article from the mold.
 41. Theprocess as set forth in claim 39 wherein said cone-shaped inlet portionhas an apex and said gas is injected into the plastic material at saidapex.
 42. The process as set forth in claim 39 further comprising thestep of holding constant the plastic material injection pressure in themold cavity for a predetermined period of time prior to the injection ofgas into the plastic material.
 43. A process for injection molding ahollow plastic tubular article comprising the steps of: (a) injecting aquantity of plastic material into a mold cavity to at leastsubstantially fill said mold cavity, the mold cavity having asubstantially cone-shaped inlet portion, wall surfaces forming anelongated central portion, and an exit portion, said plastic materialbeing injected into said cone-shaped inlet portion through a ring gatemechanism and entering said mold cavity along said wall surfacesthereof; (b) injecting pressurized gas into the plastic material in themold cavity; (c) holding the pressure of the gas and plastic in the moldcavity for a predetermined amount of time; and (d) allowing a portion ofthe plastic material in the mold cavity to be expelled into at least onesecondary cavity coupled to the mold cavity.
 44. The process as setforth in claim 43 further comprising the steps of: (e) permitting theplastic material to solidify; (f) exhausting the gas from the moldcavity; and (g) removing the plastic article from the mold.
 45. Theprocess as set forth in claim 43 wherein said cone-shaped portion has anapex and said gas is injected into the plastic material at said apex.46. The process as set forth in claim 43 further comprising the step ofholding constant the plastic material injection pressure in the moldcavity for a predetermined period of time prior to the injection of gasinto the plastic material.
 47. A process for injection molding a hollowtubular plastic article utilizing an injection molding machine with abarrel and nozzle and a mold with a mold cavity therein, the mold cavityhaving a substantially cone-shaped inlet portion, wall surfaces formingan elongated central portion and an exit portion, said method comprisingthe steps of: (a) injecting a quantity of plastic material from theinjection molding machine into the mold cavity at said cone-shaped inletportion, said plastic material entering said mold cavity along said wallsurfaces thereof; (b) injecting pressurized gas into the plasticmaterial in the mold cavity; and (c) allowing a first portion of theplastic material in the mold cavity to be expelled back into the barrelof the injection molding machine.
 48. The process as set forth in claim47 further comprising the step of holding the constant pressure of thegas and plastic material in the mold cavity for a predetermined amountof time before said first portion of the plastic material is expelledback into the injection molding machine.
 49. The process as set forth inclaim 47 wherein a predetermined amount of plastic material is expelledback into the injection molding machine.
 50. The process as set forth inclaim 47 wherein the gas is injected into the plastic material from saidexit portion.
 51. The process as set forth in claim 50 furthercomprising a ring gate mechanism for injecting the plastic material intosaid cone-shaped inlet portion.
 52. The process as set forth in claim 47wherein the step of allowing a first portion of the plastic material inthe mold to be expelled back into the barrel of the injection moldingmachine comprises opening a shut-off valve member positioned betweensaid mold cavity and said barrel.
 53. The process as set forth in claim52 wherein said valve member is included as part of the nozzle.
 54. Aprocess for injection molding a hollow tubular plastic article utilizingan injection molding machine with a barrel and nozzle and a mold with amold cavity therein, the mold cavity having a substantially cone-shapedinlet portion, an elongated central portion and an exit portion, saidmethod comprising the steps of: (a) injecting a quantity of plasticmaterial into said cone-shaped inlet portion of the mold cavity from theinjection molding machine; (b) injecting pressurized gas into theplastic material in the mold cavity; and (c) allowing a first portion ofthe plastic material in the mold cavity to be expelled back into thebarrel of the injection molding machine by opening a shut-off valvemember positioned between said mold cavity and said barrel.
 55. Theprocess as set forth in claim 54 further comprising the step of holdingthe constant pressure of the gas and plastic material in the mold cavityfor a predetermined amount of time before said first portion of theplastic material is expelled back into the injection molding machine.56. The process as set forth in claim 54 wherein a predetermined amountof plastic material is expelled back into the injection molding machine.57. The process as set forth in claim 54 wherein the gas is injectedinto the plastic material from said exit portion.
 58. The process as setforth in claim 54 wherein said valve member is included as part of thenozzle.
 59. A process for injection molding a hollow plastic tubulararticle comprising the steps of: (a) injecting a quantity of plasticmaterial to fill or substantially fill a mold cavity, the mold cavityhaving a first substantially cone-shaped inlet portion, an elongatedcentral portion and an exit portion; (b) holding the plastic materialinjection pressure constant in the mold cavity for a first predeterminedperiod of time; (c) injecting pressurized gas into the plastic materialin the mold cavity; (d) holding the pressure of the gas and plastic inthe mold cavity for a second predetermined amount of time; (e) allowinga portion of the plastic material in the mold cavity to be expelled intoat least one secondary cavity coupled to the mold cavity; (f) permittingthe plastic material to solidify; (g) exhausting the gas from the moldcavity; (h) removing the tubular-shaped plastic article from the mold;and (i) trimming at least one end of the article to form a tubulararticle of substantially constant cross-section.
 60. The process as setforth in claim 59 wherein said cone-shaped inlet portion has an apex andsaid gas is injected into the plastic material in said apex.
 61. Theprocess as set forth in claim 59 wherein said plastic material isinjected into the mold cavity from an injection molding machine with abarrel and nozzle, said method further comprising the step of allowing aportion of the plastic material in the mold to be expelled back into thebarrel of the injection molding machine.
 62. The process as set forth inclaim 59 wherein said exit portion comprises a substantially cone-shapedportion, said cone-shaped exit portion having an apex and said expulsionof plastic material from the mold cavity in the secondary cavity occursthrough said apex.
 63. The process as set forth in claim 59 furthercalculating the volume of said at least one secondary cavity in order toallow expulsion of a predetermined amount of plastic material from themold cavity.
 64. The process as set forth in claim 59 wherein the stepof allowing a portion of the plastic material in the mold to be expelledcomprises opening a valve member in a conduit connecting the mold cavitywith the secondary cavity.